This application claims the priority of German application No. 199 371 52.0, filed Aug. 6, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a combined component for afterburning anode exhaust gases from a fuel cell system and for vaporizing educts to be fed to the fuel cell system.
U.S. Pat. No. 5,229,222 discloses a fuel cell system in which the anode exhaust gas of the fuel cell is treated with the aid of an exhaust catalyzer before releasing it to the environment.
Furthermore, in hydrogen-powered fuel cell systems it is common practice to produce the hydrogen to be fed to the fuel cell by means of hydrocarbon reformation. For this purpose an appropriate hydrocarbon, methanol for example, and water are first fed to an evaporator and then reformed in a reactor.
In conventional fuel cell systems, the exhaust catalyzer for the afterburning of anode exhaust gases from fuel cell systems and the evaporator are made as separate components. As a result the space required and the total weight of these two components has been relatively great, which proves disadvantageous especially for mobile applications, such as in the automotive field.
U.S. Pat. No. 5,840,437 describes feeding the exhaust gases through a heat exchanger by which then an evaporator is heated, for example, one in which water is evaporated. However, the catalytic post-treatment of exhaust gas is not disclosed in this patent.
One object of the present invention is to provide a fuel cell system with an exhaust gas catalyzer and an evaporator which are uncomplicated, and require low expenditures in terms of cost and weight.
Another object of the invention is to provide such a fuel cell system which is especially suitable for mobile applications.
These and other objects and advantages are achieved by the combined component according to the invention for afterburning the anode exhaust gases of a fuel cell system and for evaporation of educts to be fed to the fuel cell system, in which the tubing of the evaporator is arranged within the exhaust catalyzer, advantageously in direct contact with a catalyst filling provided in the catalyzer, or by a direct coating of the tubing. In this manner an optimum heat exchange can be achieved between the two elements of the combined component. Such a combined component is overall substantially smaller and lighter than was the case in conventional solutions using separate components.
In an embodiment of the component of the present invention, the tubing of the evaporator surrounds the inner tube of the exhaust catalyzer in a substantially spiral manner. With this configuration of the tubing of the evaporator, it is possible to compensate favorably for the thermomechanical stress on the tubing in comparison with conventional evaporators (compensating thermal elongation by means of the spiral shape). The shape can in this case be chosen such that the specific heat input is adapted to the evaporation process; that is, for example, evaporator areas requiring high heat at places within the exhaust catalyzer can be arranged for a correspondingly great energy input.
It has proven advantageous, furthermore, that especially in the two-phase region of the evaporator, the centrifugal forces which are caused by the spiral shape and the velocity of the educts flowing through the evaporator, force the fluid phase against the tube surfaces. Heat exchange or transfer of heat from the tube surfaces to the liquid phase proves to be energetically more desirable than heat transfer to the gaseous phase.
Advantageously, portions of the tubing of the evaporator are made with a reduced cross section. In this way the velocity of the flow of the educts streaming through the evaporator are optimally adapted to the thermal transfer of the exhaust gas catalyzer. It is expedient to arrange a reduction of the tubing cross section, especially in the liquid phase area.
It is preferred that the cross section be reduced by means of inserts or linings placed in the tubing. Such inserts can easily be placed at the desired locations within the tubing. The inserts can be installed centrally within the tubing, for example, so that a xe2x80x9cburstingxe2x80x9d of drops of the liquid phase of the educts can be achieved. This results in an enlargement of the droplet surface area.
The inner surfaces and/or outer surfaces of the tubes are advantageously textured. By means of such a configuration, in the form of vortex tubes, for example, the transfer of heat between the exhaust gases and the educts flowing through the tubes of the evaporator are additionally improved. For example, by an appropriate texturing of the inner surfaces, turbulence can be achieved at the tubing surface in the liquid and/or gaseous phase of the educts. An appropriate texturing of the outer surfaces will positively influence the transfer of heat between the tubing and the catalyst surrounding them.
It is desirable to provide a catalytic coating on the outer surfaces of the tubes to achieve good transfer of heat to a catalytic material. Especially in the case of textured interior surfaces of the tubes, a further improved transfer of heat to the catalytically coated exterior surfaces of the tubing can be assured. If the exterior surfaces of the tubes are simultaneously textured, an enlargement of the catalytically coated area can be achieved.
According to an additional embodiment of the combined component according to the invention, the tubes are configured substantially as a set of single tubes running parallel to one another and surrounding the inner tube of the exhaust gas catalyzer in a substantially semicircular manner, and the educts can be fed through a feeder duct communicating with the single tubes, and the at least partially vaporized educts can be carried away through a outlet duct communicating with the outlet end of the single tubes. A tubing arrangement configured in this manner proves to be especially simple and inexpensive to manufacture.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings: